As a separator for a lead-acid battery (particularly a liquid lead-acid battery), there has been a separator formed of a microporous film that generally contains from 20 to 60% by weight of a polyolefin resin having a weight average molecular weight of 500,000 or more (which is generally a super high molecular weight polyethylene), from 40 to 80% by weight of inorganic powder having a specific surface area of 50 m2/g or more (which is generally silica fine powder), from 0 to 30% by weight of a plasticizer (which is generally a mineral oil), which also functions as a pore opening agent, from 0 to 10% by weight of a surfactant (solid content), and from 0 to 5% by weight of an additive (including an antioxidant, a weather stabilizer, and the like), which is referred to as a polyethylene separator.
The separator formed of a microporous film is generally a sheet having a base thickness of approximately from 0.1 to 0.3 mm, an average pore diameter (mercury intrusion method) of approximately from 0.01 to 0.5 μm, and a porosity (mercury intrusion method) of approximately from 50 to 90% by volume obtained in such a manner that a raw material composition obtained by mixing the polyolefin resin, the inorganic powder, the plasticizer (which is mixed in an amount that is larger than the aforementioned separator composition), the surfactant, and the additive is heated, melted and kneaded, and simultaneously extruded into a sheet, and rolled to a prescribed thickness, from which the plasticizer is entirely or partially removed by extraction.
In recent years, in a liquid lead-acid battery using the separator formed of a microporous film, a penetration short circuit (dendrite short circuit) phenomenon is becoming a problem, in which dendrite (i.e., a dendritic deposition of lead) penetrates through the separator in the thickness direction to connect the positive and negative electrodes, resulting in short circuit. The penetration short circuit phenomenon is such a phenomenon that is accelerated by getting the sulfuric acid electrolytic solution close to neutral due to over discharge of the battery. The background thereof includes the particular use conditions of the battery, such as an automobile with a charge controlling system and an automobile with a start-stop system, which are the recent specifications of automobiles, and the battery is generally in a use environment, in which charge and discharge are repeated without full charge, and tends to be in a rather discharged state. Accordingly, while the “agitation effect of the electrolytic solution through gassing occurring in the over charged state”, which is naturally counted in the ordinary automobile battery (i.e., a liquid lead-acid battery), cannot be expected, the stratification phenomenon of the electrolytic solution (i.e., the phenomenon, in which sulfuric acid having a high specific gravity formed on the electrode in charging moves to the lower part of the battery, causing a difference in concentration of the electrolytic solution in the vertical direction of the battery) tends to occur, and in the upper part of the battery, in which the stratification phenomenon occurs, the formation and growth of dendrite is accelerated under the use environment of a rather discharged state, due to the low concentration of the electrolytic solution, and the separator formed of a microporous film that is said to have an average pore diameter of approximately from 0.01 to 0.5 μm is said to undergo a dendrite short circuit penetrating through the separator.
Patent Document 1 proposes that in a separator for a liquid lead-acid battery, formed of a ribbed microporous film having a rib for contacting an electrode, protruding on one surface of a flat sheet, assuming that the ribbed microporous film is distinguished into a rib portion and a base portion, the porosity, the average pore diameter, and the maximum pore diameter of the rib portion are made smaller by 0.8 time or less, or the surface opening ratio thereof is made smaller by 0.8 time or less, than the base portion, thereby preventing dendrite from penetrating through the rib for contacting an electrode, and consequently a dendrite short circuit is suppressed from occurring in the separator formed of a ribbed microporous film.